The present invention relates to the investment casting of metal in a mold made using a disposable pattern, more particularly, to investment casting in a manner to improve as-cast surface finish of the cast component as well as to provide an improved pattern material.
Investment casting is widely used in the manufacture of myriad cast components including complex gas turbine engine components, such as blades and vanes made of nickel or cobalt base superalloys. In the investment casting process, a wax or other disposable pattern of the component to be cast is made typically by injecting molten wax into a pattern die cavity and solidifying the material in the die cavity. Ceramic mold material then is coated on or invested about the pattern to form a casting mold upon selective removal of the pattern by heating (melting), chemical dissolution or other conventional pattern removal technique. The ceramic investment mold typically is fired to develop mold strength, and then molten metal is cast into the mold and solidified to form the cast component, which will have the configuration of the pattern employed to make the mold.
Existing wax pattern materials normally contain a stable, solid filler material, such as for example only 4,4-isopropylindene diphenol available as Bisphenol A (BPA) or cross-linked polystyrene, which results in wax properties that limit dimensional distortion, reduce visual defects, control shrinkage, and improve dewax capabilities. Presently used filler material is a mechanically ground material that is characterized by angular surface configuration, such as an acicular particle configuration and/or fiber-like particle configuration. This filler morphology creates significant undesirable side effects which include rough and pitted casting surfaces that require extensive post-casting finishing operations and increased wax injection pressures into the pattern die cavity during pattern fabrication. Such increased wax injection pressures in the pattern die cavity can break fragile ceramic cores positioned in the die cavity and about which the wax is injected in the manufacture of wax/core pattern assemblies for use in casting hollow components, such as internally cooled turbine blades and vanes.
An object of the present invention is to provide an investment casting method conducted in a manner to improve as-cast surface of the cast component and to reduce the extent of post-casting surface finishing operations.
Another object of the present invention is to provide an improved pattern material and pattern for use in forming a refractory casting mold for use in investment casting methods.
The present invention provides an investment casting method in which a pattern material including one or more matrix constituents and substantially spherical filler particulates in a certain size range is formed into a pattern configuration of the component to be cast. The spherical filler particulate size range is selected effective to improve as-cast surface finish of the cast component by providing an improved, uniform pattern surface texture characterized by substantially reduced random, localized surface depressions and pits. The improved, uniform pattern surface is imparted to the component cast in a mold made using the pattern.
In particular, the component cast in the mold exhibits an improved as-cast surface finish with improved, much more uniform surface texture with reduced random, localized surface pitting and other gross surface defects so as to, in turn, reduce the extent of post-casting surface finishing operations. Moreover, the pattern material can be injected into a pattern die cavity at a lower injection pressure that reduces breakage of a ceramic core positioned in the die cavity in the manufacture of wax/core pattern assemblies for use in casting hollow components, such as internally cooled turbine blades and vanes.
In one embodiment of the present invention, the pattern material comprises one or more heat meltable wax and/or resin matrix constituents and substantially spherical filler particulates within a particle size range of about 10 microns to about 70 microns particle diameter effective to improve as-cast surface finish of a nickel or cobalt superalloy casting. The aforementioned objects and advantages of the present invention will become more readily apparent from the following detailed description of the invention taken with the following drawings.